如何使用 ORDER BY RAND() 优化 MySQL 查询？

MySQL RAND() 排序优化

ORDER BY RAND() 的缓慢查询带来了性能挑战。

问题

ORDER 查询BY RAND() 通常会导致性能不佳，特别是在大型或频繁更新的表上。 MySQL 的解决方案（MySQLPerformanceBlog）可能不够。

解决方案

要优化 RAND() 排序，请考虑以下方法：

SELECT  *
FROM    (
        SELECT  @cnt := COUNT(*) + 1,
                @lim := 10
        FROM    t_random
        ) vars
STRAIGHT_JOIN
        (
        SELECT  r.*,
                @lim := @lim - 1
        FROM    t_random r
        WHERE   (@cnt := @cnt - 1)
                AND RAND(20090301) < @lim / @cnt
        ) i

这种方法通过在迭代数据时保持运行概率来避免排序，使其比 ORDER BY 更高效RAND().

对于单行选择

要选择单个随机行，请尝试以下查询：

SELECT  aco.*
FROM    (
        SELECT  minid + FLOOR((maxid - minid) * RAND()) AS randid
        FROM    (
                SELECT  MAX(ac_id) AS maxid, MIN(ac_id) AS minid
                FROM    accomodation
                ) q
        ) q2
JOIN    accomodation aco
ON      aco.ac_id =
        COALESCE
        (
        (
        SELECT  accomodation.ac_id
        FROM    accomodation
        WHERE   ac_id > randid
                AND ac_status != 'draft'
                AND ac_images != 'b:0;'
                AND NOT EXISTS
                (
                SELECT  NULL
                FROM    accomodation_category
                WHERE   acat_id = ac_category
                        AND acat_slug = 'vendeglatohely'
                )
        ORDER BY
                ac_id
        LIMIT   1
        ),
        (
        SELECT  accomodation.ac_id
        FROM    accomodation
        WHERE   ac_status != 'draft'
                AND ac_images != 'b:0;'
                AND NOT EXISTS
                (
                SELECT  NULL
                FROM    accomodation_category
                WHERE   acat_id = ac_category
                        AND acat_slug = 'vendeglatohely'
                )
        ORDER BY
                ac_id
        LIMIT   1
        )
        )

此优化假设ac_ids 分布比较均匀。

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